【数据结构】双向链表

1.概念与结构

1.1概念

注意：这里的“带头”跟前面我们说的“头结点”是两个概念，实际前面的在单链表阶段称呼不严谨，但是为了更好的理解就直接称为单链表的头结点。

带头链表里的头结点，实际称为“哨兵位”，哨兵位结点不存储任何有效元素，只是站在这里“放哨的”。 

2.实现双向链表

2.1 创建双向链表结构体

typedef int LTDatatype;

typedef struct ListNode
{
	LTDatatype data;
	LTNode* prev;
	LTNode* next;
}LTNode;

2.2 创建新结点

LTNode* LTBuyNode(LTDatatype x)
{
	LTNode* newnode = (LTNode*)malloc(sizeof(LTNode));
	if (newnode == NULL)
	{
		perror("malloc fail!");
		exit(1);
	}
	newnode->data = x;
	newnode->next = newnode->prev = newnode;
	return newnode;
}

2.3 初始化

LTNode* LTInit(LTNode** phead)
{
	LTNode* phead = LTBuyNode(-1);
	return phead;
}

2.4 删除

2.4.1 头删

void LTPopFront(LTNode* phead)
{
	assert(phead);
	assert(!LTEmpty(phead));
	LTNode* del = phead->next;
	phead->next = del->next;
	del->next->prev = phead;
	free(del);
	del = NULL;
}

2.4.2 尾删

void LTPopBack(LTNode* phead)
{
	assert(phead);
	assert(!LTEmpty(phead));
	LTNode* del = phead->prev;
	phead->prev = del->prev;
	del->prev->next = phead;
	free(del);
	del = NULL;
}

2.4.3 删除指定位置结点

void LTErase(LTNode* pos)
{
	assert(pos);
	LTNode* del = pos;
	pos->prev->next = pos->next;
	pos->next->prev = pos->prev;
	free(pos);
	pos = NULL;
}

2.5 插入

2.5.1 头插

void LTPushFront(LTNode* phead, LTDatatype x)
{
	assert(phead);
	LTNode* newnode = LTBuyNode(x);
	newnode->next = phead->next;
	newnode->prev = phead;
	phead->next->prev = newnode;
	phead->next = newnode;
}

2.5.2 尾插

void LTPushBack(LTNode* phead, LTDatatype x)
{
	assert(phead);
	LTNode* newnode = LTBuyNode(x);
	newnode->next = phead;
	newnode->prev = phead->prev;
	phead->prev->next = newnode;
	phead->prev = newnode;
}

2.5.3 在指定位置之前插入

void LTInsertFront(LTNode* phead, LTNode* pos,LTDatatype x)
{
	assert(pos);
	LTNode* newnode = LTBuyNode(x);
	newnode->next = pos;
	newnode->prev = pos->prev;
	pos->prev->next = newnode;
	pos->prev = newnode;
}

2.5.4 在指定位置之后插入

void LTInsertAfter(LTNode* phead, LTNode* pos, LTDatatype x)
{
	assert(pos);
	LTNode* newnode = LTBuyNode(x);
	newnode->next = pos->next;
	newnode->prev = pos;
	pos->next->prev = newnode;
	pos->next = newnode;
}

2.6 判断是否为空

bool LTEmpty(LTNode* phead)
{
	assert(phead);
	return phead->next == phead;
}

2.7 查找

LTNode* LTFind(LTNode* phead,LTDatatype x)
{
	assert(phead);
	LTNode* pcur = phead;
	while (pcur->next != phead)
	{
		if (pcur->data == x)
		{
			return pcur;
		}
		pcur = pcur->next;
	}
	return NULL;
}

2.8 销毁

void LTNodeDestroy(LTNode** pphead)
{
	assert(pphead && *pphead);
	LTNode* pcur = (*pphead)->next;
	while (pcur != *pphead)
	{
		LTNode* Next = pcur->next;
		free(pcur);
		pcur = Next;
	}
	//摧毁哨兵位结点
	free(*pphead);
	*pphead = NULL;
	pcur = NULL;
}

void LTNodeDestory2(LTNode* phead)
{
	assert(phead);
	LTNode* pcur = phead->next;
	while (pcur != phead)
	{
		LTNode* Next = pcur->next;
		free(pcur);
		pcur = Next;
	}
	free(phead);
	phead = pcur = NULL;
}

3.顺序表与链表的分析